Are cheap Windows 10 keys legit?

It is not legit to buy a cheap Windows 10 key from such websites. Microsoft does not endorse it and will file a lawsuit against the people behind such websites if it finds out the websites that sell such keys and bulk deactivate all such leaked keys.

How much money is a Windows 10 key?

Downsides of Buying from Microsoft

Microsoft charges the most for Windows 10 keys. Windows 10 Home goes for $139 (£119.99 / AU$225), while Pro is $199.99 (£219.99 /AU$339). Despite these high prices, you’re still getting the same OS as if you bought it from somewhere cheaper, and it’s still only usable for one PC.